Carbon nanotubes are a new material that is attracting attention from numerous fields as a result of having superior electrical conductivity, thermal conductivity, mechanical strength and the like. Various studies have been conducted not only on the use of these carbon nanotubes alone, but also on their use as composite materials, in which they are dispersed in other materials. For example, a carbon nanotube dispersion, in which carbon nanotubes are dispersed in a dispersion medium, can be used as an electrical conductivity-imparting agent or an anti-static agent. In addition, the carbon nanotube dispersion can be coated on a substrate such as a film or a sheet to form an electrically-conductive layer, and this substrate can be used in an emitter material of a display, a fuel cell electrode material, a gas storage material or the like.
In general, carbon nanotubes are produced and sold in a state in which a large number of tubes are aggregated together. Carbon nanotubes in this aggregated state have the problem of being difficult to uniformly disperse in other materials (liquid media). In order to solve this problem, various methods have been investigated to improve the dispersibility of carbon nanotubes. For example, Patent Document 1 describes a method for crushing a mixture of a prescribed liquid and carbon nanotubes using a wet-medium stirring mill, and dispersing the carbon nanotubes in the liquid. Patent Document 2 describes a method for dispersing carbon nanotubes in a mixture of an amide-based polar organic solvent and a nonionic surfactant. Patent Document 3 describes a method for improving dispersibility of carbon nanotubes by attaching a conjugated polymer to a portion of the carbon nanotubes.
Patent Document 1: Japanese Patent Application Laid-Open No. 2005-324999
Patent Document 2: Japanese Patent Application Laid-Open No. 2005-075661
Patent Document 3: Japanese Patent Application Laid-Open No. 2005-089738
However, in the method described in Patent Document 1, the carbon nanotubes are susceptible to excessive fragmentation and damage to the tube wall structures. Carbon tubes that have been shortened due to excessive fragmentation or which have undergone considerable damage are unable to properly and adequately exhibit the inherent properties of carbon nanotubes (such as electrical conductivity or strength). In addition, in the methods described in Patent Documents 2 and 3, numerous types of materials (ingredients), such as surfactants or conjugated polymers, are mixed, resulting in an increase in the number of steps, such as a step for mixing these mixture materials and a step for removing unreacted substances (purification step), thereby resulting in the risk of increased complexity of the production process. In addition, the production cost of the carbon nanotube dispersion increases due to the use of such special mixture materials.
Therefore, there is a long-felt need to provide a carbon nanotube dispersion, in which the carbon materials are uniformly dispersed and a satisfactory dispersed state is maintained over a long period of time, and also inherent properties of the carbon nanotubes can be exhibited. In addition, there is a long-felt need to provide a method for producing such a carbon nanotube dispersion. Moreover, there is also a long-felt need to provide a carbon nanotube film (namely, a film having carbon nanotubes as the main constituent) obtained by applying such a carbon nanotube dispersion onto a substrate, and a film element provided with the carbon nanotube film.